1. Field of the Invention
This invention relates to non-contact, remote ultrasonic testing of railroad tracks. More particularly, Formed Laser Sources (FLS) using pulsed laser light for generation of ultrasonic stress waves are combined with air-coupled detection of ultrasound in a Laser-Air Hybrid Ultrasonic (LAHU) approach to non-contact, remote testing of railroad track.
2. Description of the Related Art
Maintenance of railroad rails is one of the greatest problems facing the transportation industry today. In one four-month period in 1998, a major railroad company experienced ten derailments due to broken rails at an expense of over $1.3 million. In its Newsletter, in September of 2000, the Texas Research Institute estimated that every ninety minutes a derailment, an accident, or any other rail related incident takes place in the US.
Despite the fact that a variety of inspection techniques have been used since the very early days of the introduction of railway, none of them is satisfactory for detection of many possible defects. The earliest of these methods, visual inspection, is obviously too slow and incapable of detecting internal defects in the rail. Furthermore, many defects on the surface of the rail are missed by the visual inspection method because of surface coverings of dirt, grease or other foreign matter.
The magnetic induction method, dating back to 1928, suffers from “liftoff” problems (magnetic field strength decreases with distance from surface of rail). It can only be used to identify surface breaking cracks.
The most current method, dating back to 1949, uses contact ultrasonic transducer sleds or piezoelectric ultrasonic transducers in a test car's rolling rubber wheels. These are filled with water or oil and in constant contact with the railroad track. These ultrasonic methods can detect both surface and internal cracks only when they are in favorable positions and orientations. They are not very effective in detecting Transverse Detail Defects (TDD), Vertical Split Heads (VSH), and rail base cracks. These three cracks are very critical and are the main cause of derailments.
Both the magnetic induction and ultrasonic methods examine only the rail top surface because of obstacles regularly appearing along the sides of the rail which fasten the rails together. In addition, these methods are currently limited to low testing speeds of about 10-15 mph.
In the U.S., the Federal Railroad Administration rules require that any indication considered suspect by the test equipment on a test car are hand verified immediately. This leads to a stop-start test mode, which effectively reduces the overall test speed in any given workday.
A more recent, non-contact means for inspecting rail track is disclosed in U.S. Pat. No. 6,324,912. It discloses the use of acoustic transducers for both the generation and detection of ultrasound on railroad tracks. The disclosed method relies on Doppler shifted frequencies caused by the high-speed motion between the rail and the source of the ultrasonic wave. The technique is estimated to be limited to operating velocities of the source of the ultrasonic waves that are above 65 mph and has been demonstrated only on a smooth laboratory spinning wheel.
Other air-coupled ultrasonic generation techniques rely on the resonance modes associated with the different cross-sections of the rail. Up to four hundred measurements are collected and averaged in order to obtain a discernable signal. The technique imposes a static condition between the probes and test specimen in order to perform averaging calculations while retaining phase of the resonant frequency. In addition, ultrasonic source air-coupled transducers are sensitive to their position and orientation with respect to the surface of the specimen. They often impose test configurations that place the test probes in positions that compromise their safety. These configurations are generally rejected by the railroad industry.
Frequency control of laser generated acoustic signals has been previously accomplished by temporal or spatial modulation. However, such temporal modulation requires a very high repetition rate, which would then translate to lower laser pulse energy. The resulting acoustic signals are too weak for most industrial applications.
Spatial modulation has been accomplished through the use of shadow masks, diffraction gratings, corrugated lenticular arrays or other means. All of these techniques produce specific illumination patterns and have low efficiency in delivering the laser energy to a surface in which defects are sought to be detected.
These techniques differ from those of the present invention. In the present invention, a series of high energy lenses are shaped in order to maximize the delivery of the laser energy to the targeted surface and to allow flexibility in shaping the illuminated region to produce optimum ultrasonic signals for the desired flaw detection capabilities.
The present inventors' early experiments utilizing non-contact, ultrasonic techniques for the inspection of railroad tracks were initially disclosed in May 2000. See “Non-Contact Ultrasonic Inspection of Railroad Tracks,” 45th International SAMPE Symposium, San Diego, Calif., May 21-25, 2000. The teachings and disclosure of this work is hereby incorporated by reference. The experimental techniques of this early work used laser beams focused to a point. These early techniques will be seen to differ significantly from those revealed herein.
The present inventors have also documented much of the work that is disclosed herein in the scientific literature. See “Laser-Based and Air Coupled Ultrasound as Noncontact and Remote Techniques for Testing Railroad Tracks,” Materials Evaluation, vol. 60(1), January 2002, pp. 65-70; “Point and Line Source Laser Generation of Ultrasound for Inspection of Internal and Surface Flaws in Rail and Structural Materials,” Research in Nondestructive Evaluation, vol. 13(4), December 2001, pp. 189-200, “Narrowband Laser-Generated Surface Acoustic Waves Using A Formed Source In The Ablative Regime,” Journal of Acoustical Society of America, to be published, Spring 2003, “Laser-Based and Air Coupled Ultrasound as Noncontact and Remote Techniques for Testing Railroad Tracks,” (Translated to Italian), The Journal of the Italian Society of Nondestructive Testing Monitoring Diagnostics, vol. 23(2), 2002, pp. 34-41, “Laser And Air-Coupled Transducer For Non-Contact Ultrasonic Inspection In The Railroad Industry (in Italian),” ENEA Trisaia Research Center (MT) Italy, AIPnD (Italian Society for Nondestructive Testing), To-Be Published 2003, “Laser-Air Hybrid Ultrasonic Technique for the Inspection of Rail Steel,” 11th International Symposium on Nondestructive Characterization of Materials, Berlin, Germany. Jun. 24-28, 2002, (in publication), and “Sensitivity of Point And Line Source Laser Generated Acoustic Wave To Surface Flaws,” IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, To Be Published 2003. The teachings and disclosure of these works are hereby incorporated by reference.
With the rising cost of transportation and increasing number of drivers on the road, the use of passenger and cargo trains is becoming more attractive. However, before increasing the speed and axle load on railroad tracks, faster and more reliable inspection methods ate still needed in order to prevent property damage and life-threatening injury.
3. Objects and Advantages
There has been summarized above, rather broadly, the prior art that is related to the present invention in order that the context of the present invention may be better understood and appreciated. In this regard, it is instructive to also consider the objects and advantages of the present invention.
It is an object of the present invention to provide an improved testing method for inspecting railroad tracks.
It is another object of the present invention to provide a non-contact and remote testing method that can detect hard to find surface breaking and internal cracks, vertical and traverse cracks and other material discontinuities in railroad tracks.
It is yet another object of the present invention to provide a testing method that enables one to generate and detect ultrasonic signals on the base of the rail, web of the rail and from the side of the rail while still keeping all instrumentation and equipment remote and non-contact above the top surface of the rail.
It is a further object of the present invention to provide a rail track testing method and apparatus that utilizes Formed Laser Sources (FLS) methodology to enable one to control the wavefront and frequency of the laser-generated acoustic signal so as to yield optimum interaction with and detection of rail defects such as cracks.
It is a still further object of the present invention to provide a rail track testing method and apparatus that utilizes Laser-Air Hybrid Ultrasonic (LAHU) techniques and FLS sources to detect various types of critical cracks that cause rail reliability problems and are difficult or impossible to detect via conventional means.
It is an object of the present invention to provide a rail track testing apparatus and method that enables inspection of the complete rail and successfully detect cracks and other defects at test speeds exceeding those currently available to the railroad industry.
These and other objects and advantages of the present invention will become readily apparent as the invention is better understood by reference to the accompanying summary, drawings and the detailed description that follows.